Red Hat Enterprise Linux (RHEL) is a Linux distribution developed by Red Hat and targeted toward the commercial market. Red Hat Enterprise Linux is released in server versions for x86, x86-64, Itanium, PowerPC and IBM System z, and desktop versions for x86 and x86-64. All of the Red Hat's official support and training, together with the Red Hat Certification Program, focuses on the Red Hat Enterprise Linux platform. Red Hat Enterprise Linux is often abbreviated to RHEL, although this is not an official designation.[4]

The first version of Red Hat Enterprise Linux to bear the name originally came onto the market as "Red Hat Linux Advanced Server". In 2003 Red Hat rebranded Red Hat Linux Advanced Server to "Red Hat Enterprise Linux AS", and added two more variants, Red Hat Enterprise Linux ES and Red Hat Enterprise Linux WS.

Red Hat uses strict trademark rules to restrict free re-distribution of their officially supported versions of Red Hat Enterprise Linux,[5] but still freely provides its source code. Third-party derivatives can be built and redistributed by stripping away non-free components like Red Hat's trademarks, including community-supported distributions like CentOS and Scientific Linux, and commercial forks like Oracle Linux, which aim to offer 100% binary compatibility with Red Hat Enterprise Linux.

Red Hat Enterprise Linux Server subscription is available at no cost for development purposes.[6] Developers need to register for the Red Hat Developer Program and agree to licensing terms forbidding production use. This free developer subscription was announced on March 31, 2016.

There are also "Academic" editions of the Desktop and Server variants. They are offered to schools and students, are less expensive, and are provided with Red Hat technical support as an optional extra. Web support based on number of customer contacts can be purchased separately.[7]

It is often assumed the branding ES, AS, and WS stand for "Entry-level Server", "Advanced Server" and "Work Station", respectively. The reason for this is that the ES product is indeed the company's base enterprise server product, while AS is the more advanced product. However, nowhere on its site or in its literature does Red Hat say what AS, ES and WS stand for.

In Red Hat Enterprise Linux 5 there are new editions that substitute former Red Hat Enterprise Linux AS/ES/WS/Desktop:[8][9]

Originally, Red Hat sold support for versions of Red Hat Linux (Red Hat Linux Enterprise Edition 6.2E was essentially a version of Red Hat Linux 6.2/7 with different support levels.)[11] Starting with RHEL 2.1 AS in 2002, Red Hat sold their first version of RHEL. It was based on Red Hat Linux, but used a much more conservative release cycle. Later versions included technologies from the Red Hat–sponsored Fedora community distribution project. Red Hat Enterprise Linux release schedules do not follow that of Fedora (around 6 months per release) but are more conservative (2 years or more).

Fedora serves as upstream for future versions of RHEL. RHEL trees are forked off the Fedora repository, and released after a substantial stabilization and quality assurance effort.[12] For example, RHEL 6 was forked from Fedora at the end of 2009 (approximately at the time of the Fedora 12 release) and released more or less together with Fedora 14. By the time RHEL 6 was released, many features from Fedora 13 and 14 had already been backported into it. The Fedora Project lists the following lineages for older Red Hat Enterprise releases:[12]

(Note about Fedora Core 1 and Red Hat Enterprise Linux 3: Red Hat released Red Hat Linux 10 beta 1, then took two forks from that codebase to seed both Fedora Core 1 and Red Hat Enterprise Linux 3 beta releases. There was some cross-pollination between the two up until shortly before the first production RHEL 3 release. Therefore, both FC1 and RHEL3 came from a common fork of RHL10beta1.)

In addition, the Fedora project includes Extra Packages for Enterprise Linux (EPEL), a community-provided set of packages for RHEL going beyond the ones that Red Hat selected for inclusion in its supported distribution. The Fedora project provides the following explanation:[14]

Both Fedora and Red Hat Enterprise Linux are open source. Fedora is a free distribution and community project and upstream for Red Hat Enterprise Linux. Fedora is a general purpose system that gives Red Hat and the rest of its contributor community the chance to innovate rapidly with new technologies. Red Hat Enterprise Linux is a commercial enterprise operating system and has its own set of test phases including alpha and beta releases which are separate and distinct from Fedora development.

Originally, Red Hat's enterprise product, then known as Red Hat Linux, was made freely available to anybody who wished to download it, while Red Hat made money from support. Red Hat then moved towards splitting its product line into Red Hat Enterprise Linux which was designed to be stable and with long-term support for enterprise users and Fedora as the community distribution and project sponsored by Red Hat. The use of trademarks prevents verbatim copying of Red Hat Enterprise Linux.

Since Red Hat Enterprise Linux is based completely on free and open source software, Red Hat makes available the complete source code to its enterprise distribution through its FTP site to anybody who wants it. Accordingly, several groups have taken this source code and compiled their own versions of Red Hat Enterprise Linux, typically with the only changes being the removal of any references to Red Hat's trademarks and pointing the update systems to non-Red Hat servers. Groups which have undertaken this include CentOS (the 8th most popular Linux distribution as of November 2011),[15]Oracle Linux, Scientific Linux, White Box Enterprise Linux, StartCom Enterprise Linux, Pie Box Enterprise Linux, X/OS, Lineox, and Bull's XBAS for high-performance computing.[16] All provide a free mechanism for applying updates without paying a service fee to the distributor.

Rebuilds of Red Hat Enterprise Linux are free but do not get any commercial support or consulting services from Red Hat and lack any software, hardware or security certifications. Also, the rebuilds do not get access to Red Hat services like Red Hat Network.

Unusually, Red Hat took steps to obfuscate their changes to the Linux kernel for 6.0 by not publicly providing the patch files for their changes in the source tarball, and only releasing the finished product in source form. Speculation suggested that the move was made to affect Oracle's competing rebuild and support services, which further modifies the distribution. This practice however, still complies with the GNU GPL since source code is defined as "[the] preferred form of the work for making modifications to it", and the distribution still complies with this definition.[17] Red Hat's CTO Brian Stevens later confirmed the change, stating that certain information (such as patch information) would now only be provided to paying customers to make the Red Hat product more competitive against the growing number of companies offering support for products based on RHEL. CentOS developers had no objections to the change since they do not make any changes to the kernel beyond what is provided by Red Hat.[18] Their competitor Oracle announced in November 2012 that they were releasing a RedPatch service, which allows public view of the RHEL kernel changes, broken down by patch.[19][20]

A number of commercial vendors use Red Hat Enterprise Linux as a base for the operating system in their products. Two of the best known are the Console Operating System in VMware ESX Server and Oracle Linux respin.

Third-party RPM packages for Red Hat Enterprise Linux, CentOS, and Scientific Linux can be downloaded from RepoForge (formerly RPMforge),[21] an alliance of developers who previously had separate distribution sites (similar to the RPM Fusion merger).

Red Hat Enterprise Linux 7 (Maipo) is based on Fedora 19, upstream Linux kernel 3.10, systemd 208, and GNOME 3.8 (rebased to GNOME 3.14 in RHEL 7.2).[24][25] The first beta was announced on 11 December 2013,[13][26] and a release candidate was made available on 15 April 2014.[27] On 10 June 2014 Red Hat Enterprise Linux 7 was officially released.[28]

The life cycle of Red Hat Enterprise Linux is at least seven years for versions 3 and 4, while it spans at least 10 years for more recent versions 5, 6 and 7. The life cycle comprises several phases of varying length with different degrees of support. During the first phase ("Production 1"), Red Hat provides full support and updates software and hardware drivers. In later phases ("Production 2" and "Production 3") only security and other important fixes are provided and support for new hardware is gradually reduced.[32]

In the last years of the support lifecycle (after seven years for the version 4 and earlier, or after 10 years for the version 5 and later), critical and security-related fixes are only provided to customers who pay an additional subscription ("Extended Lifecycle Support Add-On") that is available for versions 3, 4 and 5, and covers a limited number of packages.[33]

To maintain a stable application binary interface (ABI), Red Hat does not update the kernel version, but instead backports new features to the same kernel version with which a particular version of RHEL has been released. New features are backported throughout the Production 1 phase of the RHEL lifecycle.[37] Consequently, RHEL may use a Linux kernel with a dated version number, yet the kernel is up-to-date regarding not only security fixes, but also certain features.[38] One specific example is the SO_REUSEPORTsocket option which was added to Linux kernel 3.9, and was subsequently backported and became available since RHEL 6.5, which uses version 2.6.32 of the Linux kernel.[39][40][41]

The Extended Update Support (EUS) allows the organization / company that has deployed RHEL to choose when they wish to deploy to the new minor version. For the first 6 months of the EUS channel / yum repo , features may be added, then the channel is locked down so the only bug and security fixes will be patched. The organization / company has 24 months to move to a new EUS branch. EUS allow the organization / company to stay pinned on a minor version if a requirement of 3rd party application is only tested with a particular minor version of RHEL, such as Oracle Database , IBM DB2 , IBM Cloud Orchestrator, etc. There may be an extra cost associated by using the EUS repos/channels depending on the agreement that the organization / company has with Red Hat.[42] For more information what is Included/Excluded from EUS see .[43]